Network availability is of significant importance in tele- and datacommunication networks evolving nowadays. One way of improving the availability of such networks comprises building protection features into the networks such that efficient means are provided to switch traffic to a different path in the case of a failure somewhere in a path used. With the rapid development of DWDM (Dense Wavelength Division Multiplexing) and of general and special techniques of building optical networks using e.g. different forms of WDM (Wavelength Division Multiplexing), there is a growing interest in means to handle protection in optical transmission systems and optical networks.
In FIGS. 1a-1c schematics of optical networks are shown which have various levels of ring protection in the optical layer. Thus, the illustrated networks all have a ring structure and contain Optical Add and Drop (Add/Drop) Multiplexer (OADM) blocks 1, also called optical add and drop nodes or add/drop nodes, which contain the filters and couplers necessary to add, drop and block wavelengths which are terminated in the node. Each such OADM block 1 is connected to a left OADM block and to a right OADM block through pairs 3 of optical fibers, one pair outgoing from the considered node in a left hand or western direction and a second pair outgoing in a right hand or eastern direction. In the scheme of FIG. 1a each OADM block 1 is connected to a transmitter-responder or transponder (TP) 5 and to a receiver (R) 7 through an optical switch 9. The transponder 5 transmits the wavelength signal in both directions and a block receiving the wavelength signal can choose the direction from which to receive that wavelength channel using its receiver 7 by setting its switch 9 accordingly. The receiver 7 receives light signals and converts them to for example electrical signals. In the network as illustrated in FIG. 1a there are two possible different paths of transmitting information from one node to another way, a first path extending in a clockwise direction and a second path extending in counter-clockwise direction. Both paths can be used simultaneously, the first path for some channels and the second for other channels. However, normally only one of the two possible paths will be used for all communication from one node to another node. When a fault occurs in such path, the other path can be used, this feature providing the protection of the network. Such a protected network can handle single faults in an optical fiber, in the cable holding the pair of fibers and connecting the OADM blocks or in the OADM blocks. In a special kind of control of such networks there is always one inactive link between two adjacent blocks whereas all the other links are used for transmission. The position of the inactive link can then be displaced when a fault occurs.
The optical network as illustrated by the scheme of FIG. 1b gives the same level of protection as that of FIG. 1a but may allow a more efficient use of transmitter power and a reuse of wavelengths in the ring architecture. Here also the transponders 7 are connected to the OADM block 1 through optical switches 11, allowing the direction to be chosen, in which the respective transponder will transmit. There may, however, also be concerns about the reliability of the transmitter optical switches 11 and the possibility to monitor the health of the protection path. In the optical network of FIG. 1c separate transponders 5′, 5″ are provided for transmitting in each direction, this layout not requiring any transmitter optical switches. In this third network scheme also faults in a transmitter or in a transponder can be mitigated.
It is a feature common to all the schemes as discussed above that a switching function on the receiving end is required in order to choose the direction from which the wavelength is to be received (the switch 9). A natural conclusion is then to let a simple optical space switch handle this function which may be an efficient solution in the type of WDM systems which have up to now been introduced on the market. These systems are, however, primarily intended for long distance applications and the system architectures are typically based on optical amplifiers as fundamental building blocks and a separate wavelength channel is typically used for supervisory signalling. In metropolitan and wide area networks, which are more short haul type applications by their nature, other more cost efficient system and technology solutions have to be found, while the important system functions still have to be retained. These solutions would then preferably not be based on optical amplifiers what implies that it will be imperative to minimize the attenuation between all ports in the node. Furthermore, it becomes important to take into account all the network functions that need to be implemented in connection with the “optical switch” used (e.g. the switch 9 in FIGS. 1a-1c). One such natural way to implement the receiving end is illustrated by the schematic block diagram of a node or OADM block of FIG. 2, only showing the devices necessary for receiving in the node.
The wavelength channels from other OADM blocks, arriving to the considered node from the left and the right directions respectively, arrive at a left input fiber 21 and a right input fiber 23 as illustrated in FIG. 2. From these signals a portion of the optical power is extracted using optical tapping couplers 25, 27 connected to the respective input fiber. The extracted signals are fed to optical-to-electrical converters 29, 31 converting the instant optical power to electric power representing the optical signal. The average power or the power levels of the two wavelength channels can then be measured as indicated by the outputs 33, 35. Also, an overlaid embedded supervisory data channel can be detected in the electric signals by feeding the detected instant power signal to a supervisory channel receiver or supervisory channel receivers 37. The detected power levels at the outputs 33, 35 are used to monitor the health of the paths from the left and the right direction respectively and to make decisions about when and how to protect the node changing the position of an optical switch 39. This optical switch corresponds to the switch 9 of FIGS. 1a-1c. 
Since a separate supervisory wavelength channel would be significantly more costly, both in terms of component cost and additional attenuation in the node, such an embedded channel solution is to be preferred. The other output ports of the tapping couplers 25, 27 are connected to the optical switch 39. The position of the switch 39 determines the direction from which the wavelength channel is to be received. The output of this switch 39 is fed into another optical tapping coupler 41 which has one output connected to another optical-to-electrical converter 43 providing an electric signal at an output 45, from which the average power of power level at the output of the optical switch 39 can be detected and monitored. By comparing the power levels as represented by the electric signals on the outputs 33, 35, 45 of the power detectors 29, 31, 43 the health of and the attenuation in the switch 39 can be deduced. Another output 46 from the tapping coupler 41 is intended to be connected to the client receiver (the receiver 7 in FIGS. 1a-1c).
The implementation in FIG. 2 may be natural as well as economically and technically feasible. There are however a number of important issues which need to be considered using this type of implementation based on an optical switch:
The reliability of the switch. This switch is a single point of failure in the link and hence the reliability of the switching component is very important. Unfortunately it is difficult to test the long term reliability of many of the optical switches available on the market.                An associated issue is that it does not appear to be easy to reliably health monitor an optical switch. How do you know that it will actually switch in a proper way when required?        The penalty in terms of signal attenuation associated with the optical couplers and the optical switch.        The cost of the optical switch and the cost of additional optical couplers and detectors required for extracting monitoring and supervisory information.        How and where to extract a supervisory channel and how to block this channel from passing to the client layer, possibly disturbing the detection of the client signal.        
Finally there are other issues which may indirectly have an impact on the choice of implementation such as for instance that the receivers in the client equipment may be unsuitable for directly receiving a wavelength channel from an optical DWDM network. This may be due to poor receiver sensitivity, dynamic range problems or that the receiver incorporates a detector which cannot handle the used wavelength.
In the published European patent application No. 0 689 309 an add/drop node of a two-fiber ring network using a time division transmission method is disclosed. In selecting a light signal from one of the parallel paths a switch or a receiving control section can be used. The receiving control section appears to make “physical grouping and distribution of signal lines” and it is said to have the function of “allocating a differing number of channels to the clockwise and counterclockwise directions” and it “controls what signal from either the clockwise or counterclockwise transmission path is to received”. Thus, no details are disclosed on the internal structure of the receiving control section. In the published European patent application No. 0 668 674 a fiber network having add/drop nodes is disclosed.